What is the criteria for classifying sols? What type of sols are known?

Sols are classified based on their affinity for the dispersed phase for the dispersion medium. There are types of sols – lyophobic and lyophilic.

What is the difference between lyophilic and lyophobic sol?

A sol in which colloidal particles have an affinity for the dispersion medium is called lyophilic sol whereas, in a lyophobic sol, colloidal particles have no or very little affinity for the dispersion medium.

What is meant by ‘dialysis’?

The phenomena of purification of sols by passing through a semipermeable membrane is known as dialysis.

What is the special characteristic of the paper used in dialysis?

It only permits the electrolyte to pass through it and doesn’t allow the passage of colloidal particles.

What is the nature of charge on (i) ferric hydroxide and (ii) arsenious sulphide sols?

Can sulphuric acid (H2SO4) be used for preparing the original solution?

No, because sulphuric acid (H2SO4) will produce sulphates of the cations and some sulphates like lead sulphate (PbSO4) are insoluble. So, we may not get the required clear original solution.

Explain how Aluminium (III) [Al3+] can be confirmed using cobalt nitrate.

Take a salt solution and add cobalt nitrate to it. Burn a filter paper which has been dipped into this solution. If you detect blue colour on black ash of filter paper, Aluminium (III) [Al3+] confirmed.

NH4Cl is added to prevent precipitation of radicals other than III group as hydroxides by common ion effect. If we add NH4OH first, radical of group IV, V & VI will also precipitate and accordingly we cannot do it.

Why is it essential to boil off hydrogen sulphide [H2S] gas before precipitating III group?

If H2S isn’t boiled off group IV cations will also get precipitated.

How does the addition of ammonium hydroxide (NH4OH) help in precipitation of cations as sulphides?

Solubility product of Group IV sulphides is greater than those of group IV radicals. Thus, a higher concentration of S2- ions are required to precipitate Group IV radicals as sulphides. OH– ions of ammonium hydroxide (NH4OH) with H+ ions of H2S, shift the equilibrium towards right or increasing the concentration of S2- ions.

Black precipitates in Group IV indicate which cations?

Presence of either Ni2+ or Co2+.

Why can sodium hydroxide [NaOH] not be used instead of ammonium chloride [NH4Cl] and ammonium hydroxide (NH4OH)?

NaOH is a very strong electrolyte and its ionization cannot be supressed. Moreover, it dissolves precipitates of Al3+, and Cr3+ as a result of formation of AlO2-, CrO42- complexes.

Potash alum, a double salt, commonly known as ‘fitkari’ has the formula K2SO4.Al2(SO4)3.24H2O. It can be prepared by making an equimolar solution of potassium sulphate and aluminium sulphate in the minimum amount of water. A few ml of diluted H2SO4 is added to prevent the hydrolysis of Al2(SO4)3.18H2O. Cooling of the hot saturated solution yields colourless crystals of Potash alum.

Mohr’s salt i.e. ferrous ammonium sulphate [FeSO4.(NH4)2SO4.6H2O] is a double salt. It can be prepared by making an equimolar solution of hydrated ferrous sulphate and ammonium sulphate in a minimum amount of water. A few ml of diluted H2SO4 is added to prevent the hydrolysis of FeSO4.7H2O. Cooling of the hot saturated solution yields lights green crystals of Mohr’s salt.